1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for generating, acquiring and processing marine seismic data and, more particularly, to mechanisms and techniques for separating seismic signals recorded by streamers and generated by plural marine seismic sources.
2. Discussion of the Background
Marine seismic data acquisition and processing may be used to generate a profile (image) of the geophysical structure under the seafloor (subsurface). While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface is important, for example, to those who need to determine whether the oil and gas reservoirs are located.
The traditional marine seismic acquisition uses a seismic source, such as an air gun or air gun array, to generate acoustic energy. The seismic source is towed by a vessel under water, and the generated acoustic energy propagates toward the subsurface where the energy is reflected from various formations of the subsurface. The same vessel or another vessel tows one or more streamers. A streamer includes one or more seismic sensors (receivers) that are configured to record the seismic wave reflected by the various formations in the subsurface. The recorded waves are then used to generate the profile of the subsurface.
For marine applications, seismic sources are essentially impulsive (e.g., compressed air is suddenly allowed to expand under water). One of the most used sources is air guns. The air guns produce a high amount of acoustic energy over a short time. Such a source is towed by a vessel either at the water surface or at a certain depth. The acoustic waves from the air guns propagate in all directions. A typical frequency range of the acoustic waves emitted by the impulsive sources is between 6 and 300 Hz. However, the seismic waves generated by two air guns cannot be distinguished (during processing) because they have the same signature. This characteristic of the impulsive sources reduces their effectiveness, as discussed next.
FIG. 1 shows a system 10 in which a source array 20 is towed under water with plural streamers 30 (four in this case). The figure illustrates a cross-sectional view of this system, i.e., a plane perpendicular to the streamers. Seismic waves 22a-d emitted by the source array 20 are reflected from an interface 40 in the subsurface, and the reflected waves are recorded by receivers of the streamers 30. A distance “a” between two successive reflections is called a bin size. Because this bin size is measured along a cross-line, “a” represents the cross-line bin size. The cross-line is defined as a line substantially perpendicular to the streamers, different from an axis Z that is also perpendicular on the streamers, but describes the depth of the streamers under water. An inline is a line that extends substantially along the streamers and is perpendicular on the cross-line. For example, the Cartesian system shown in FIG. 1 has the X axis parallel to the inline, the Y axis parallel to the cross-line, and the Z axis describes the depth of the streamers.
With this arrangement, the cross-line bin size is half the cross-line distance 42 between two consecutive streamers. It is noted that the streamers are typically placed 100 m from each other and, thus, a typical cross-line bin size is 50 m. The inline bin size may be much smaller, as it depends mainly on the separation between the receivers in the streamer itself, which may be around 12 to 15 m. With a cross-line bin size on the order of 50 m, aliasing effects may be produced, especially for the highest frequencies, because the maximum bin size is inversely proportional to the frequency. In other words, the illumination of the subsurface on the cross-line is poor. Thus, it is desired to decrease the cross-line bin size during a seismic survey.
A common technique for reducing the cross-line bin size is the use of multiple source arrays that employ a flip-flop acquisition scheme. In this mode, the vessel tows two sources 20 and 20′ as shown in FIG. 2. Two sources are shown for simplicity, but more than two sources may be used to increase the coverage. This system 50 is configured to shoot the sources in a flip-flop manner, i.e., shot first one source 20 while the second source 20′ is inactive and then, after a period time for listening and recording, the second source 20′ is shot while the first source 20 is inactive. The listening period extends for a predetermined time so that the reflections corresponding to the emitted waves from the first source 20 are recorded and the seismic waves generated by the first source 20 die down). Another period of listening and recording follows for the second source. Then, the process is repeated and this flip-flop process constitutes a firing sequence. This scheme doubles the coverage lines of the subsurface as new reflections points are introduced by the second source 20′, thus reducing the cross-line bin size to a distance “b”, which is smaller than “a”.
However, the flip-flop sequence has the following limitation: because there is a time delay (in the order of seconds) between firing the first source and the second source, there are gaps in the data recorded by the streamers. To illustrate these gaps, FIG. 3 shows a top view the two sources 20 and 20′ and the streamers 30. Line 40 illustrates the subsurface portion that reflects the seismic waves generated by source 20 to streamer 30a, and line 42 illustrates the subsurface portion that reflects the seismic waves generated by the source 20′ to the same streamer 30a. It is noted that the dash portions of each line 40 and 42 correspond to waiting times, i.e., no recordings, and the solid portions of these lines correspond to recording times.
In other words, when source 20 is active and source 20′ is inactive, portion 40a of line 40 is actively sampled with acoustic waves and those waves are recorded, while portion 42a of line 42 is not sampled by source 20′. As the number of sources is increased, for example, up to four, the waiting times need to be increased. This results in the increase of portion 42a (i.e., gap in the recordings). When this portion reaches around 150 m, which is the case for four sources, the gaps in the recorded data are considered unacceptable. Thus, simply increasing the number of sources, although may reduce the cross-line bin size, is not feasible.
Instead of increasing the number of sources for achieving a smaller cross-line bin size, it is possible to increase the number of streamers or to reduce the cross-line distance between the streamers. However, these solutions are not favored by the industry as (1) the price of a streamer and the price for towing the streamer is much higher than towing a source, (2) the drag created by increasing the number of streamers dramatically increases, and (3) a coverage area by each pass of the vessel during the survey is reduced.
Thus, there is a need in the industry to provide a solution that decreases the cross-line bin size, reduces the cost of the survey and also improves the quality of the final image.